Actually, what you should do is what i just did. Just put multiple pictures into the one file, and you can put up to 10 files in 4096 colors, or like 15 in the 256 colors or somethin. The site that you download them from http://www.kenwood.net-disp.com/i/05products/xxv_01d/wallpapers.html, doesn't have the KDC-x990 listed, but the XXV-01D uses the same files that the 990 does. So either just compile a bunch of pictures you have already, or just download some from that site. That site has some pretty cool ones too.
Hope that helps, with you posting about it made me look into it more and sure enough it works. Oh, and if you notice the picture goes negative everyonce in awhile, don't panic, it kind of a built in screensaver if you will. But just go to standby and turn it off and the title will automatically scroll so some movement is going on in the screen.